Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, is typically used to generate a circuit pattern to be formed on an individual layer in an IC. This pattern is transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To increase production rate of scanned patterns, a patterning device, e.g., a mask or reticle, is scanned at constant velocity, for example 3 meters/second across a projection lens, back and forth along a scan direction. Therefore, starting from rest, the reticle quickly accelerates to reach the scan velocity, then at the end of the scan, it quickly decelerates to zero, reverses direction and accelerates in the opposite direction to reach the scan velocity. The acceleration/deceleration rate is, for example, 15 times the acceleration of gravity. There is no inertial force on the reticle during the constant velocity portion of the scan. However, the large inertial force encountered during the acceleration and deceleration portions of the scan, for example 60 Newtons (=0.4 kg of reticle mass×150 m/sec2 of acceleration) can lead to slippage of the reticle. Such slippage can result in a misaligned device pattern on a substrate. Attempts to solve misalignment include using a clamp to hold the reticle in place and/or using a friction coating to increase friction between the reticle and the clamp. Ever increasing production rates demand ever faster direction reversals, and therefore higher accelerations have reduced the benefits of these solutions.
Other attempts to solve reticle slippage include devices that are mounted on a chuck of a reticle stage to exert a force between the reticle and the chuck equal in magnitude and opposite in direction to the inertial force experienced by the reticle during the acceleration portion of the motion of the stage. External power or inertial masses and levers are used to produce the force.
Externally powered anti-slip devices dissipate power near the reticle so they can cause thermal stability problems. Also, wires can dynamically couple the chuck to a moving frame, which is undesirable for vibration isolation. Separate control requires extra electronic hardware and software.
Inertial anti-slip devices can add substantial mass to the chuck, which is undesirable for high acceleration. Inertial anti-slip devices can further complicate the dynamics of the chuck by adding low eigenmodes that require additional filtering of an input motion command signal.